Anti-GBM nephritis, the prototypic autoimmune nephritis for which the kidney target antigen is well characterized, is a key system for discovery involving human kidney disease pathogenesis. Nephritogenic epitopes reside on alpha3 (IV) NC1 collagen within the glomerular basement membrane. Compelling recent discoveries indicate the existence of a crossreactive and previously unsuspected additional self-antigen involved in disease pathogenesis, as well as immunological links between diverse anti-collagen diseases. These findings indicate remarkable complexity in autoimmune disease regulation, the elucidation of which will provide new insights into disease onset, suppression, and arrest. The goals of this proposal are to identify this second antigen and the molecular basis of novel receptor-ligand interactions, and to explore their engagement in human autoimmunity and disease pathogenesis in vivo. This effort relies on cutting edge but validated technologies and cross-disciplinary collaborations. Specific Aim 1 will use state-of-the- art and complementary proteomics approaches to identify the unknown second antigen that engages and regulates pathogenic reactivity to alpha3(IV)NC1 collagen. Specific Aim 2 will use innovative computational prediction modeling to determine receptor-ligand structure, both to further inform Aim 1 and to provide new insight into potential environmental disease precipitants and overlapping autoimmune regulatory circuits. Specific Aim 3 will develop a humanized model to examine autoimmune responses in vivo in the context of a human immune system, using the NOD-scid-gamma strain for enhanced engraftment of hematopoietic cells. The model will also generate unique human immune reagents and tools, with the ultimate goal of providing a platform for preclinical testing to validate research findings and to test immune modulating interventions in vivo.